In general, in an inductively coupled reactor, radio frequency (RF) plasma source power is applied to an antenna set outside an evacuated chamber, and power is inductively coupled from the antenna through an RF window, e.g., a dielectric material, forming a part of the aforementioned evacuated chamber. In this way, the plasma can be maintained. In such a reactor, the plasma can be maintained at a high ion density at a relatively high vacuum (about 10.sup.-3 Torr). Consequently, the direction of the ions incident to the substrate under processing is predominantly perpendicular to the substrate surface. As a result, RF plasma source power can be of a high frequency. The high ion density provides high-speed processing. When the inductively coupled plasma is used for the dry etching, bias RF power must be applied to the substrate to control ion energy. An RF bias current flows from the substrate into the plasma through the cathode sheath and then flows from the plasma into the wall of the evacuated chamber through the anode sheath. In this case, since the area of the evacuated chamber acting as the anode is substantially larger (e.g., 3-5 times) than the area of the susceptor electrode acting as the cathode, the potential difference between the plasma and the evacuated chamber is relatively small, while the potential difference between the plasma and the cathode is relatively large. Consequently, the energy of the ions impinging on the substrate is relatively large, leading to a high etching efficiency. Furthermore, since the voltage on the side of the anode sheath is relatively low, ion sputtering of the evacuated chamber, contamination caused by the products of such sputtering and other adverse effects are inhibited.
Because the induced electromagnetic field cannot pass through the interior of a typical conductor, a dielectric (insulator) such as fused silica and ceramic has been used as the RF window of the inductively coupled plasma reactor. Because the RF bias current does not flow through this part, its area is not included in the area of the anode. Consequently, when the area of the RF window is enlarged to accommodate a large coil antenna for processing a large wafer, the anode/cathode area ratio is reduced, thereby reducing the desired effects discussed above.
Referring now to the conventional inductively coupled plasma reactor of FIG. 1, a hemispherical RF window 100 is positioned facing a substrate 105 supported on a wafer pedestal 110, and a helical coil antenna 115 is set outside RF window 100, being wound in a helix whose axis of symmetry coincides with the axis of symmetry of the hemispherically shaped RF window 100. A conductive (metallic) cylindrical side wall 120 supports the hemispherical RF window 100. The wafer pedestal 110 can support an electrostatic chuck 125 for holding the wafer 105 in place. An RF plasma source power supply 130 furnishes RF plasma source power, for example at 13.56 MHz, to the coil antenna 115 through an RF impedance match circuit 135. An RF plasma bias power supply 140 furnishes RF plasma bias power to the wafer pedestal 110 through an RF impedance match circuit 145. The chamber side wall 120 is grounded to provide an RF return for the bias power applied to the wafer pedestal 110.
Since RF window 100 is a dielectric material, the anode consists solely of the conductive (metallic) wall 120 of the evacuated chamber, which unfortunately presents a relatively small area to the plasma. In this case, the area of the anode is almost as small as that of the cathode. As a result, the potential difference between the plasma and the cathode becomes so small that the etching rate is reduced. On the other hand, the potential difference between the plasma and the anode is large. As a result, ion sputtering of the chamber wall 120 is more intense so that wear of the evacuated chamber wall 120 and consequent metal contamination on the substrate 105 are aggravated.
Typically, the RF window 100 consists solely of a dielectric such as alumina. The area ratio of the top surface (the portion acting as an electrode) of the wafer pedestal (susceptor) 110 to the chamber wall 120 is about 2.0. (Interestingly, the area ratio of the susceptor 110 to the RF window is about 4.5.)
Table I shows the relationship between the self-bias voltage V on the substrate or silicon wafer 105 and the bias power density (W/cm.sup.2). In the example of Table I, the internal pressure is kept at 10 mTorr, an etching gas mixture of 5 parts CHF3 and 1 part Ar for etching a silicon oxide film is fed into reaction chamber from a gas inlet 150 at a gas flow rate of 200 sccm, and RF plasma source power of 3 kW is applied to the antenna 115 to maintain a plasma.
TABLE I ______________________________________ Bias Power Density (W/cm.sup.2) Self Bias Voltage on Wafer (volts) ______________________________________ 0.5 150 1.0 260 2.0 440 3.0 620 4.0 800 ______________________________________
In this example, the self-bias voltage is the time average of the bias voltage generated on the silicon wafer 105 capacitively connected to the wafer pedestal (susceptor electrode) 110 through the insulation layer of the electrostatic chuck 125. In this example, when ion scattering is negligible under a relatively low pressure, the self-bias voltage approximately corresponds to the average energy of the ions accelerated from the plasma to silicon wafer 105.
Etching of the silicon oxide film requires bombardment of the ions with relatively high kinetic energy to induce the etching reaction. In the foregoing example, a self-bias voltage of 600 V is needed. Consequently, as can be seen from Table I, a bias power density of about 3 W/cm.sup.2 is needed for conventional dry etching device having a purely dielectric RF window 100. Because the working area of the upper surface of the wafer pedestal (susceptor electrode) 110 must be larger than that of wafer 105, the area of the susceptor electrode is in the range of 300-400 cm.sup.2 for a 200 mm diameter silicon wafer. Consequently, as can be seen from Table I, when the area of the susceptor's upper surface is 400 cm', it is necessary to apply a total bias power of 1.2 kW between the susceptor electrode 110 acting as the cathode and the chamber wall 120 acting as the anode.
When such a high bias power is applied, the temperature of the silicon wafer 105 rises significantly. The etching rate of the photoresist film used as the etching mask increases, and the selection ratio decreases. In addition, because the bias power with 30-40% of the energy applied to the antenna 115 is consumed in heating the plasma, the electron temperature tends to rise.
In order to increase the anode/cathode area ratio to ameliorate the foregoing problems, the chamber volume can be enlarged by increasing its height or radius of evacuated chamber. But, this expedient requires a vacuum pump with a large exhaust capacity.